Venezuela has revoked the licences of dozens of radio stations as part of a wider crackdown which could jail people deemed guilty of "media crimes" for up to four years.

At least 13 stations went off the air over the weekend and another 21 were expected to follow soon in an effort by President Hugo Chávez to extend his socialist revolution.

The move followed last week's introduction of a draft law to jail journalists and broadcasters who "harm the interests of the state", "cause panic" or "disturb social peace". Critics denounced the moves as a recipe for censorship. "What we are witnessing is the most comprehensive assault on free speech in Venezuela since Chávez came to power," said Jose Miguel Vivanco, Americas director at Human Rights Watch.

"With the exception of Cuba, Venezuela is the only country in the region that shows such flagrant disregard for universal standards of freedom of expression."

Several hundred protesters held a vigil outside one Caracas station, CNB 102.3 FM, for its final transmission on Saturday. "This is a government attack," said the station director, Zaira Belfort. "We want to keep living in democracy, and once again they've silenced us."

The government's telecommunications agency said it would revoke the licences of up to 240 radio stations, almost 40% of the total, citing irregular paperwork. It said the closures were lawful and that most radio stations remained in private hands.

Chávez applauded the decision and said the licences would be "given back to the people" in the form of broadcasters who shared his leftist vision.

He has also threatened to shut Globovision, the last outspoken opposition TV channel, claiming it was part of a "media war" backed by United States imperialists.

After a decade in power the former tank commander remains popular for his anti-poverty programmes but has tightened his grip in the wake of mounting economic problems.

Last month the government yanked opposition TV and radio adverts off the air and proposed compelling cable channels to carry Chávez's speeches live.

Critics have been most alarmed by the attorney general's announcement last week that freedom of expression "must be limited" with a yet-to-be defined bill on "media crimes".